There are various systems using TDMA (time division multiple access) for mobile communications, for instance, EGSM (extended global system for mobile communications) and DCS (digital cellular system) widely used mostly in Europe, PCS (personal communications service) widely used in the U.S., and PDC (personal digital cellular system) used in Japan. Conventionally proposed as small, lightweight, high-frequency circuit devices adapted to pluralities of systems are, for instance, dual-band, high-frequency switch modules for use in mobile communications equipments adapted to two systems of EGSM and DCS, triple-band, high-frequency switch modules for use in mobile communications equipments adapted to three systems of EGSM, DCS and PCS, etc. Wireless LAN data communications according to the IEEE802.11 standard, etc. are widely used at present, and this wireless LAN standard includes pluralities of standards having different frequency bands, etc. Various high-frequency circuits are used in wireless-LAN, multiband communications apparatuses.
When one cell phone is used in pluralities of frequency bands, the cell phone should have a high-frequency switch module comprising a circuit for branching transmission signals and received signals in pluralities of frequency bands transmitted from and received by an antenna, and a switch for switching a transmission path and a reception path. The high-frequency switch module, a key part for multiband, wireless communications, is strongly demanded to be made smaller and higher in performance, and the removal of noises in unnecessary bands is indispensable.
To meet such demand, JP 11-27177 A proposes a high-frequency switch comprising filters whose number is reduced by utilizing floating capacitance generated between an antenna and the ground to remove harmonic distortion generated in a transmission signal from a power amplifier. This high-frequency switch has an attenuation pole in a third-harmonic band by adjusting floating capacitance generated between an antenna terminal, transmission circuit terminals and receiving circuit terminals and the ground, and adjusting the length of transmission lines constituting choke elements to substantially λ/6. Although a specific adjusting method is not described, it is extremely complicated to adjust all of these floating capacitances while adjusting the transmission lines to desired length. Although attenuation should be conducted to a level of about seventh harmonics, for instance, in a GSM system, this method cannot attenuate such high-order harmonics because it uses only an attenuation pole of third harmonics.
JP 2003-69362 A proposes a diplexer comprising a parallel resonance circuit having a resonance frequency equal to the frequency of second harmonics on the side of a lowpass filter terminal to remove second harmonics efficiently. With the parallel resonance circuit connected to common terminal and the lowpass filter terminal, this diplexer efficiently removes harmonics having two times as large frequency as a first frequency on the low-frequency side, while reducing signal loss. Specifically, the lowpass filter circuit in the diplexer comprises two parallel resonance circuits constituted by coils and capacitors, with grounded capacitance connected to a connecting point of these parallel resonance circuits and the lowpass filter terminal. However, two parallel resonance circuits make the diplexer larger with a more complicated circuit structure. Also, a multi-stage lowpass filter provides large insertion loss, deteriorating the characteristics. In addition, because the lowpass filter circuit in this diplexer is designed to attenuate two times as large frequency as the first frequency, or a second frequency, it cannot sufficiently attenuate unnecessary bands other than n-th order harmonics.
When both circuits for EGSM and DCS in different frequency bands of 900 MHz and 1800 MHz, respectively, are used, interference occurs to cause the leakage of signals and the deterioration of isolation characteristics. This problem becomes conspicuous as high-frequency devices are made smaller. Against this backdrop, JP 2001-352202 A proposes a high-frequency switch module handling pluralities of transmission/receiving systems having different passbands, in which one transmission/receiving system and the other transmission/receiving system are disposed in different regions of a laminate in a plane direction. However, their formation in different regions of the laminate in a plane direction does not provide sufficient shielding between one transmission/receiving system and the other transmission/receiving system.
With respect to a lowpass filter used in a high-frequency switch module, JP 2004-328136 A proposes a lowpass filter comprising a series resonance circuit and a parallel resonance circuit connected in series for improving the attenuation characteristics of second and third harmonics in signal waves (fundamental waves), with grounded capacitances connected to both ends of the parallel resonance circuit, and with a phase-adjusting transmission line disposed between the series resonance circuit and the parallel resonance circuit. However, the attenuation characteristics and insertion loss of this lowpass filter are not necessarily sufficient to the demand of higher performance accompanied by increase in the number of bands, etc. Also, when composite filters each comprising a lowpass filter and a notch filter are connected via transmission lines, unnegligible deterioration of characteristics occurs due to electromagnetic interference and parasitic capacitance between circuits, and between filter-constituting transmission lines and capacitances. In addition, composite filters sacrifice the demand of miniaturization to improve characteristics. In the course of increasing the integration of laminate modules, it is difficult to design the arrangement of elements such as inductors, etc. constituting lowpass filters and laminate modules comprising them, which meets the demands of smaller size and higher performance.